Remove references to `tf.contrib.rnn.RNNCell` in the RNNCore docstring.
Remove references to `tf.contrib.rnn.RNNCell` in the RNNCore docstring.

While RNNCore used to inherit from RNNCell, it doesn't any longer and the docstring needed updating.

PiperOrigin-RevId: 223176347

deepmind/sonnet,deepmind/sonnet